In a system having a large number of heat generating devices, such as electronic devices, a cooling apparatus must be provided in the system to maintain the performance of the electronic devices. For example, a large scale electronic system such as a semiconductor device test system has a large number of electronic devices, i.e., VLSI and LSI circuits. Because of the increase in density and speed of the recent electronic devices, heat dissipated from the electronic devices are increasing. Examples of devices which generate a large amount of heats are ECL gate arrays, general purpose CMOS LSIs, power supply modules, and the like.
As shown in FIG. 6, electronic devices are usually mounted on a printed circuit board in the test system. Since the electronic devices are becoming smaller and smaller, a large number of electronic devices are mounted in a small area of the printed circuit board, which further increases the overall amount of heat generated by the devices.
Life times of semiconductor devices such as LSIs and gate arrays are dependent upon inner temperature (junction temperature) of the semiconductor devices. The higher the temperature, the shorter the life times become. Therefore, the semiconductor devices must be cooled to maintain the junction temperature lower than, for example, 80.degree. C. The present invention is directed to a cooling apparatus for heat generating electric devices.
As noted above, semiconductor devices are mounted on a printed circuit board. Packages of semiconductor devices which dissipate large amount of heat are made of materials of high thermal conductivity such as ceramic. The packages of the semiconductor devices are provided with a large number of electrode pins of various types. For example, a BGA (Ball Grid Array) package has ball shaped pins so that the package is surface mounted on a printed circuit board. Semiconductor devices mounted on a printed circuit board have differences in height because of the differences in size or mounting conditions. For example, 100 LSI devices mounted on a printed circuit board of 380 mm by 450 mm, there are differences of .+-.1 mm in the height among the devices. Thus, a cooling apparatus for a large number of semiconductor devices must have capability of efficiently cooling the devices having differences in height.
FIGS. 4 and 5 show examples of cooling apparatuses in the conventional technology. The example of FIG. 4 includes nozzles 26, exhaust means 22, elastic heat conductors 25, heat conductor plates 24, and metallic bellows 23. Electric devices 12, which are heat generating devices, are mounted on a printed circuit board 10 while establishing electric contact therebetween through pins 13. The elastic heat conductors 25 are arranged on the electric devices 12 to establish mechanical contact therebetween. The heat conductor plate 24 is provided between the bellows 23 and the elastic heat conductor 25.
Cooling medium, such as cooling fluid, is ejected from the nozzles 26 toward inner spaces formed by the bellows 23 and the heat conductors 24. The used cooling medium is exhausted through the exhaust means 22. Hence, the heat generated by the electric devices 12 are transmitted to the heat conductor plates 24 through the elastic heat conductors 25. The heat is then carried away by the cooling medium through the exhaust means 22. This example of cooling apparatus using the metallic bellows and cooling medium has disadvantages in that an overall cost is high and it is difficult to decrease an overall size.
The example of cooling apparatus shown in FIG. 5 includes a cooling plate 30 having cooling medium paths 31 and heat conductive springs 60. The heat conductive springs 60 are placed between the cooling plate 30 and the electric devices 12 mounted on the printed circuit board 10. Since the spring 60 mechanically contact with both the electric devices 12 and the cooling plate 30, the heat from the electric devices 12 is transmitted to the cooling plate 30. The heat is then carried away by the cooling medium flowing through the paths in the cooling plate 30.
The example of FIG. 5 using the heat conductive springs 60 and cooling plate 30 has a disadvantage wherein electric insulation between the electric devices and the cooling apparatus is insufficient. The cooling efficiency and electric insulation will be improved by providing thermal conductive grease on the contact surfaces of the electric devices 12 and the cooling plate 30 as well as heat conductive springs 60 to reduce thermal resistance among the members. However, the oil in the heat conductive grease will evaporate because of the heat. Thus, the heat transmission and electric insulation effects will be diminished.
As explained above, the cooling apparatus using the metallic bellows 23 and the cooling medium has a difficulty in reducing the size and cost. The cooling apparatus using the heat conductive spring 60 and the cooling plate 30 cannot be used for cooling the electric devices having power sources or electrodes exposed to an atmosphere because the cooling apparatus does not have the insulation capability. Moreover, the evaporation of oil in the heat conductive grease due to the high temperature causes degradation of heat conductive effects as well as contamination in inside and outside of the semiconductor test system.